A known scanning antenna is disclosed in PCT Application Number WO 91/17586 having first and second, flat plate waveguides connected by a waveguide bend. Waveforms at a focus of the first waveguide propagate as a beam of plane waves having a nonplanar, thin cylindrical, phase front, and become redirected by a parabolic profile of the waveguide bend to a beam for propagation in the second waveguide having an array of antenna apertures to be illuminated by the redirected beam.
In the known antenna, the waveguide bend has a parabolic profile that redirects the nonplanar phase front in reverse, by a 2 .pi. change in phase, to propagate in the second waveguide. The parabolic profile of the waveguide bend has a detrimental effect on the redirected phase front. A parabolic profile must be selected so as to provide a redirected beam pattern that may vary from, a narrow beam pattern with low side lobes, characteristic of a deep parabolic profile, to a broader beam pattern of more uniform illumination of the array of antenna elements with higher scattering, characteristic of a shallow parabolic profile. Accordingly, attainment of a redirected beam with a planar phase front is difficult to attain by a waveguide bend with a parabolic profile redirecting a beam having an nonplanar phase front.
In the known antenna, the first waveguide has parallel metal plates filled therebetween with dielectric material. The dielectric material extends from the focus of the waveguide to the parabolic waveguide bend. The dielectric material is without a desired feature that would emanate a planar phase front toward the waveguide bend. A wave of planar phase front would simplify the waveguide bend to a planar profile, and would reduce the detrimental effect of a parabolic waveguide bend on the redirected phase front.